Rope for a climber

[BetaHelix]

I'm looking at using this rope for climbing. The rope says not for lifting, but is rated for 90lbs, would this be a problem? We would be using 4 strands, 4x90=360 so it would support 2.5 times our robot weight.

[notmattlythgoe]

Originally Posted by BetaHelix

I'm looking at using this rope for climbing. The rope says not for lifting, but is rated for 90lbs, would this be a problem? We would be using 4 strands, 4x90=360 so it would support 2.5 times our robot weight.

This is the stuff you want.

[Bruceb]

ya we used 7/64th's Dynema cable. 1600 lb test and no stretch. very abrasion resistant.

[Kevin Sevcik]

Just be aware that Spectra and Dyneema are slipperier than "normal" ropes like nylon. Thoroughly strength test your knots to make sure the won't slip.

[dkavanagh]

We used this:
http://www.amazon.com/Braided-diamet...C CV4PTRV7JJ5

It was quite sturdy and held up well.

We also used a 400lb test kevlar line that worked, but it frayed way too easily and couldn't be trusted.

[Cory]

Originally Posted by Kevin Sevcik

Just be aware that Spectra and Dyneema are slipperier than "normal" ropes like nylon. Thoroughly strength test your knots to make sure the won't slip.

The knot also dramatically reduces the working load of the line. There are certain knots that are better than others from this standpoint.

[shewejff]

Originally Posted by notmattlythgoe

This is the stuff you want.

I second the Spectra cord. It's great stuff. We are using the 2.5 mm diameter for climbing and 3 mm diameter to stop our catapult. Both were also purchased through Amazon.

As for the comment about the knots slipping, we haven't found that at all. We don't do any crazy complicated knots, and the cord holds the knots very well.

[BetaHelix]

What motors are you using for your climbers? Would 1-2 775pros work if we have a ratchet for support?

[Cory]

Originally Posted by BetaHelix

What motors are you using for your climbers? Would 1-2 775pros work if we have a ratchet for support?

any motor is going to work if geared properly and given enough time. The amount of time you want to lift in is going to bound the problem for you.

[pilleya]

Originally Posted by BetaHelix

What motors are you using for your climbers? Would 1-2 775pros work if we have a ratchet for support?

The Linear Motion tab, in the JVN calculator

http://www.chiefdelphi.com/media/papers/2755

Should help you find the necessary gear reduction, pulley size etc. As Cory stated it is all dependent on the amount of time you wish to take to climb.

As an example one 775pro in a 64:1 VP with a 1 inch diameter spool(not taking into account spool size increase as cord is winded) should be easily be able to lift a 140 pound robot in under 3 seconds

[EricH]

You know, there's a couple of "advanced" topics you may want to research on your own time.

Static loading and dynamic loading. The 90 lb is likely a static load situation. Your 150 lb robot is most definitely a dynamic load situation.

Static: Not moving, no "extra" forces. (Just to pick an example, you park yourself on the end of a rope rated for your weight and very gently remove the support you're standing on.)
Dynamic: Motion involved, "extra" forces from various factors. (Going with the previous example, you jump up and grab onto the same rope--or use it to stop a fall, your choice. BTW, don't try this at home, I don't expect it'll end well.)

Long story short, something that's rated at 150 lb might not hold up when 150 lb is lifting itself--you might want 300 lb, 450 lb, even 600 lb. Maybe more. And 4 strands of 90-lb rope may in theory be 360 lb, but even the rig technique you use can drop that.

I could go more in-depth, but this has the makings of a) off-topic and b) fun offseason learning project. Talk to your (mechanical) engineering mentors about trying this sort of thing out or running the numbers.

[DaveL]

Originally Posted by Cory

The knot also dramatically reduces the working load of the line. There are certain knots that are better than others from this standpoint.

The rule of thumb I've heard as a sailor is that in typical cordage applications knots will reduce available strength by about 50%.
There are many factors that will affect this result. The linked research on different knots, found that well tied knots of different varieties will reduce strength less than the rule of thumb. The paper didn't mention what the knots were tied to, but my guess is something round. If you attach cordage to a sharp edge, then your resulting system strength is likely to be much less. http://itrsonline.org/wordpress/wp-c...hards.2004.pdf

Also the cordage used in the above test is quite a bit different than spectra.

Also note that if you redirect the rope's direction with a carabiner, it is going to be under greater strain than it would be in a straight line application.

All in all, sand out sharp edges where your rope is attached or contacts a surface and use rope that is much stronger than what you think is needed.
Dave